1. Field of the Invention
The present invention concerns an abrasion and/or scratch resistant article comprising an impact resistant photochromic polyurethane coating, in particular a photochromic ophthalmic lens, a process of preparation thereof without the need for high curing temperatures and long curing times, an impact resistant photochromic polyurethane film and a curable coating composition for the preparation thereof.
2. Description of Related Art
The phenomenon of photochromism has been known for many years. A compound is described as photochromic when, for example, this compound, irradiated with a light beam containing some wavelengths in the ultraviolet region, changes colour and returns to its original colour when the irradiation ceases.
There are many applications of this phenomenon, but one of the most useful known applications is in the ophthalmic optics field, in the manufacture of lenses or spectacle glasses, so as to filter the light radiation as a function of its intensity.
The incorporation of photochromic compounds into an organic material constituting an ophthalmic lens gives a glass, the weight of which is considerably less than that of conventional lenses composed of inorganic glass containing silver halides as photochromic agents.
A widely used method for manufacturing ophthalmic lenses from photochromic organic materials is the method known as “thermal transfer” (imbibition), in which the organic photochromic compounds, such as spirooxazines or chromenes, are applied to the lens by means of a temporary support such as a varnish, then the coated lens is heated so as to cause the transfer of the photochromic compound of the varnish onto the main face of the lens. This method is especially disclosed in patents U.S. Pat. Nos. 4,286,957 and 4,880,667.
Another technique known as “cast-in-place” consists of incorporating the photochromic organic compounds into a polymerizable mixture leading to a transparent organic material, introducing this into a mould and then initiating its polymerization.
After removal from the mould, a photochromic ophthalmic lens is obtained, the photochromic pigments of which are incorporated into the bulk of this lens.
This latter technique has two major disadvantages: i) it requires considerable quantities of photochromic pigments, which are costly compounds; ii) the initiators used for the polymerization degrade the photochromic pigments, which tends to cause a reduction in their photochromic performance. In addition, the presence of the degradation products, which are generally coloured, can change the appearance of the final lens, rendering it unsuitable for its use.
In addition, in the two above techniques, since the performances of the photochromic pigments are closely dependent on the material in which they are incorporated, it has been necessary to develop specific organic materials suitable for the incorporation of such pigments.
Finally, some organic materials used in ophthalmic optics such as polycarbonates (thermoplastic materials generally transformed by injection moulding) have a polymer matrix which is unsuitable for being imbibed by photochromic compounds.
An advantageous technique which is an alternative to the thermal transfer and cast-in-place techniques involves applying to the surface of a preformed ophthalmic lens a coating composition containing dissolved photochromic compounds capable to form a material that can host photochromic compounds.
The nature of the material constituting the ophthalmic lens onto which the photochromic coating is applied is thus, in principle, no longer relevant. Such a technique is disclosed for example in the patent EP 146136.
The activation and deactivation time of the photochromic compound are dependent on the free volume in the polymer matrix: the more free volume the polymer matrix has, the faster the photochromic compound can darken and fade. The free volume of the polymer matrix being dependent upon the flexibility of the chain segments of the polymer environment surrounding the photochromic compound, soft and flexible polymer matrixes such as polyurethane matrixes are preferred.
Photochromic polyurethane coatings are described in patents or patent applications U.S. Pat. No. 6,187,444 (WO 98/37115), U.S. 2005/0233153 and U.S. Pat. No. 6,773,108.
U.S. Pat. No. 6,187,444 relates to photochromic polyurethane coating compositions comprising, in a suitable solvent, at least one photochromic compound, a polyol, a blocked polyisocyanate monomer and a tin catalyst. Said coating composition is applied by spin coating onto the surface of an ophthalmic lens substrate and then cured. However, this technique cannot be implemented on certain substrates, since the blocked polyisocyanates require to be heated during the curing step to elevated temperatures (90-200° C., typically 140° C.) to react in the same way as the original free polyisocyanates. Such unblocking temperature is much higher than the Tg (glass transition temperature) of lens substrates such as certain polythiourethanes or polyepisulfides, which can cause the lens to soften and distort. Deformation of the lens alters its optical properties.
U.S. Patent Application No 2005/0233153 discloses a method for manufacturing a plastic photochromic lens, which comprises preparing a liquid composition by dissolving in a suitable solvent a thermoplastic polyurethane polymer, at least one isocyanate terminated polyurethane pre-polymer, at least one photochromic compound and optionally stabilizers, then coating this liquid composition, evaporating the solvent at a temperature chosen in the range from 60 to 130° C. to form a photochromic polyurethane film of 5-150 μm thickness. The dried film is finally cured at room temperature for 1 day to 1 week and then at a temperature chosen in the range from 50 to 130° C. for 8 hours to 1 week.
U.S. Pat. No. 6,773,108 describes a lens coated with an elastomeric film having at least one photochromic dye dispersed therein. The film is obtained by dissolving the at least one photochromic compound in a coating solution comprising an elastomeric polyurethane pre-polymer, optionally an acrylate monomer and a solvent. Incorporation of polyfunctional alcohols in the solution is also envisioned, however, the disclosed alcohols are not polyols but functionalized mono-alcohols having at least one alkoxide group, such as 2-butoxy ethanol (glycol ether) or polypropylene glycol methyl ether. Said coating solution is applied onto a lens and then air dried until tack free (for about 30 minutes), or air dried for 30 minutes and heat-dried at 95-100° C. for an additional 30 minutes to obtain a tack-free surface. This causes the polyurethane pre-polymer to form on the lens an elastomeric film possessing the beneficial characteristics of impact resistance and abrasion resistance.